1. Field of the Invention
The invention relates to immunostimulatory sequences in DNA. The invention further relates to recombinant expression vectors for use in gene therapy.
2. History of the Related Art
Recombinant expression vectors are the tools which researchers and clinicians use to achieve the goals of gene therapy and gene immunization. In gene therapy, viral and non-viral vectors are used to deliver an expressible gene into a host to replace a missing or defective gene, or to otherwise supply the host with a therapeutically beneficial polypeptide. In gene immunization, mostly non-viral vectors are used to induce an immune response by the host to an expressed antigen.
One of the obstacles to successful clinical practice of both gene therapy and gene immunization has been the often transient nature of the gene expression achieved in vivo. Transient gene expression is less problematic in gene immunization, where immune responses sufficient for certain immunization schemes may be stimulated by even short-term exposure to expressed antigen. In addition, several options are available to boost the host immune response to antigen, including use of the vector itself as an adjuvant for the desired immune response by exposing the host to non-coding, immunostimulatory nucleotide sequences (ISS-ODN) present in the vector (Sato, et al., Science, 273:352-354 (1996)).
However, in a gene therapy protocol, premature loss of gene expression deprives the host of the potential benefits of the therapy (Friedmann, Scientific American, "Making Gene Therapy Work" (June 1997)). Repetitive dosing to extend exposure of the host to a therapeutic polypeptide can require that different vectors be used to deliver each dose so the host immune response to vector antigens is minimized (Tripathy, et al., Nature Med., 2:545-550 (1996)).
One potential source of vector immunogenicity are ISS-ODN in the genome of the microbial species used to construct recombinant expression vectors. To explain, the CpG motifs which characterize ISS-ODN are present in bacteria and viruses (including retroviruses) at a much greater frequency than is seen in vertebrate genomes. One consequence of ISS-ODN activity is the ISS-ODN induced production of cytokines such as interferon-.alpha. (INF-.alpha.), INF-.gamma. and interleukin-12 (IL-12). This ISS-ODN induced inflammation is believed to be defensive against microbial infection in vertebrates and is also believed to be produced in response to ISS-ODN introduced into a host as oligonucleotides or as part of a recombinant expression vector.